This is directed to azimuthally sensitive resistivity logging devices in the context of a measuring while drilling (MWD) system. It provides a set of toroids installed for measurement from a drill collar supported MWD system which provides both a conventional axisymetric current sensitivity as well as azimuthally focused directional sensitivity.
The present disclosure is particularly adapted for use in a MWD system mounted in the sidewall of a drill collar. A drill collar is a joint of drill pipe which has an extra thick wall where the present apparatus is normally installed. This disclousure sets forth an antenna system (several coils) which cooperates with the resistivity measuring apparatus so that the received current can be focused in alternate formations. The teachings of J.J. Arps in U.S. Pa. No. 3,305,771 define the current or conventional method. It is however, a set of coils which can be operated so that, in one connection, the current flow through the formation is in a horizontal plane (normal to a vertical well) and symmetrically in all azimuthal directions around the tool axis. In another mode of operation of the equipment, the current flow through the formation is sensed in a particular azimuthal direction. By the appropriate combination of coil connections described hereinafter, the present system so deploys the toroid coils that differential signals are taken between currents measurements at selected windows or locations on the drill collar and a highly sensitive response is obtained. This sensitivity can be used to determine the particular location of the wall borehole because it provides information about the location of the drill collar relative to adjacent conductive rock beds, particularly in deviated wells, and ultimately even in horizontal drilling of a deviated well. For descriptive purposes, the tool is set forth in a vertical well and references to horizontal and vertical should be viewed in that context. This does not preclude use in a deviated well, or even in a horizontal well.
The method described in U.S. Pat. No. 4,786,874 contemplates the asymetrical generation of current in the formation by a current electrode place on one side of the drill collar, and the sensing of formation asymetrical voltage distribution by a voltage sensing electrode, with both electrodes placed on an insulated section of the drill collar. The insulated section of drill collar is a severe drawback in the disclosed system because it is structurally fragile and capable of breaking during drilling. By contrast, the present invention attacks and solves this problem by generating an axisymetric current flow from a transmitting toroid spaced at some distance from the receiver coils described hereinafter, and sensing the perturbations of the current flow caused by asymetry in the formation conductivity wherein the perturbations are sensed by the unique receiver coil configuration.
The present disclosure sets out a window array which preferably has two coils thereon where the window array is approximately rectangular in shape. The top and bottom edges of the rectangular window coil toroids are arranged at the same azimuth on the drill collar. The remaining two sides of the window coil toroids are vertical along the drill collar, thus defining a four sided window permitting current to flow through the window. This kind of construction with appropriate switching provides a plurality of coil segments which can be connected to form a current focusing window in the form of a four sided window which directs current flow through the adjacent formations based on the geometry and azimuth of that particular window. By means of a signal combining circuit, the coils can be rearranged cooperative with other and similar coil defined windows at the same or different locations on the drill collar so that conventional current flow patterns can occur. This is particularly helpful where a deviated well is drilled. Ordinarily, a well is drilled vertically for at least a portion of its depth. After vertical drilling has penetrated the earth to a certain depth, it is then often necessary to deviate the well. To illustrate the point, assume that a well is drilled vertically for several thousand feet and is then deviated to a horizontal direction. When it is vertical, it is typically normal to the geological formations traversed by the borehole. The present apparatus is able to operate when the wells are vertical. The current is injected in all directions through the adjacent formations to provide resistivity measurements of the respective formations penetrated by the well borehole. However, when the well is deviated to horizontal, it may be necessary to guide the horizontal drilling process so that the horizontal portion is formed entirely within a single formation. Assume that the formation has definitive top and bottom boundaries with electrical resistivity different from that of the formation, and is of substantial interest for possible production. In that case, the horizontal drilling process may involve guiding the well during drilling to assure that it is approximately centered between the top and bottom boundaries of the formation. If the formation is thirty feet thick, this provides a quite narrow target for the horizontal portion of the well borehole. In this instance, it is very helpful to convert from sensing current flow patterns which are symmetrical in all directions to a focused azimuthal current sensing mode of operation. The present apparatus contemplates a dual mode of operation so that the toroids arranged around the drill collar sense current in preferred azimuthal directions thereby providing a focus which is able to indicate resistivity along particular directions. One important benefit of this is the ability to distinguish changes in resistivity as the well deviates within the formation of interest and begins to approach either the upper or lower boundaries thereof. The current flow region is thus distorted in a particular azimuthal direction, yielding a measure of resistance contrast, and that measure is tied to the precise position of the measuring equipment at the instant of that current reading so that the resistivity data indicates the location of the interfaces defining the formation of interest. An example will be given so that this will become more clear hereinafter. The virtues just mentioned represent features which are an advance over the art, one representative structure being the ELIAS tool of Bureau de Recherches Geologiques et Minieres. This also is an advance over U.S. Pat. No. 4,786,874 which discloses a system showing a MWD tool having isolated current injection electrodes 30 at specific locations thereon.
The present apparatus also incorporates a method of operation which permits simultaneous measurement of uniform current injection in axisymetric directions and one which is specific for a particular azimuth.